linux-kernel - Re: Problem with WARN_ON in mutex

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Date:   Tue, 10 Dec 2019 22:53:56 +0100
From:   Peter Zijlstra <peterz@...radead.org>
To:     Sebastian Andrzej Siewior <bigeasy@...utronix.de>
Cc:     Thomas Gleixner <tglx@...utronix.de>,
        David Howells <dhowells@...hat.com>,
        Davidlohr Bueso <dave@...olabs.net>,
        Ingo Molnar <mingo@...nel.org>, linux-kernel@...r.kernel.org,
        Frederic Weisbecker <fweisbec@...il.com>
Subject: Re: Problem with WARN_ON in mutex_trylock() and rxrpc

On Tue, Dec 10, 2019 at 09:32:25PM +0100, Sebastian Andrzej Siewior wrote:
> On 2019-12-10 20:25:38 [+0100], Peter Zijlstra wrote:
> > AFAICT the only assumption it relies on are:
> > 
> >  - that the softirq will cleanly preempt a task. That is, the task
> >    context must not change under the softirq execution.
> > 
> >  - that the softirq runs non-preemptible.
> > 
> > Now, both these properties are rather fundamental to how our softirqs
> > work. And can, therefore, be relied upon, irrespective of the mutex
> > implementation.
> 
> softirq is preemptible on -RT (I think you know that already but just in
> case).

Indeed, but there it also runs in task context and then it all works
naturally. The !preempt thing is required for when it runs on top of a
task; then it functions as a priority ceiling like construct.